The handler method either needs to take a parameter matching the type of event that is generated (ActionEvent for onAction), or no parameters. This is the cause of the warning you're getting in Eclipse. Since you don't need to access the event itself, just define the method to take no parameters:

@FXML
public void dateTimePicker(){
// ...
}

Finally, if you want to show a new window, you have to create a new window. So: